EJB持久性

EJB2.0中使用的实体bean持久化机制在很大程度上被EJB3.0取代。现在实体bean是一个简单的POJO映射表。

以下是持久性API的关键角色

• Entity - 持久对象代表数据存储记录。这也是可序列化的。

• EntityManager - 持久性接口做数据操作，如添加/删除/更新/找到持久化对象（实体）。它还有助于执行查询使用query接口

• Persistence unit (persistence.xml) - 持久性单元介绍了持久性机制的属性。

• Data Source (*ds.xml) - 数据源描述了数据存储相关的属性，如连接URL。用户名，密码等。

为了证明EJB的持久化机制，我们要做好以下几项工作。

• Step 1. 在数据库中创建表.

• Step 2. 创建实体类对应的表.

• Step 3. 创建数据源和持久性单元

• Step 4. 创建一个无状态EJB EntityManager实例.

• Step 5. 更新无状态EJB。添加添加记录并获得通过实体管理器从数据库中记录的方法。

• Step 6. 一个基于控制台应用程序客户端将访问无状态EJB的持久化数据库中的数据。

创建表

创建一个表 books 在默认的数据库 postgres.

CREATE TABLE books ( id     integer PRIMARY KEY, name   varchar(50) );

创建实体类

//mark it entity using Entity annotation  //map table name using Table annoation @Entity @Table(name="books") public class Book implements Serializable{ private int id; private String name; public Book(){ } //mark id as primary key with autogenerated value //map database column id with id field @Id @GeneratedValue(strategy= GenerationType.IDENTITY) @Column(name="id") public int getId() { return id; } ... }

创建数据源和持久性单元

DataSource (jboss-ds.xml)

xml version="1.0" encoding="UTF-8"?>   PostgresDS jdbc:postgresql://localhost:5432/postgres org.postgresql.driver sa sa 5 20 5  

Persistence Unit (persistence.xml)

 version="1.0" xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd">  name="EjbComponentPU" transaction-type="JTA"> java:/PostgresDS false    name="EjbComponentPU2" transaction-type="JTA"> org.hibernate.ejb.HibernatePersistence java:/PostgresDS false   name="hibernate.hbm2ddl.auto" value="update"/>   

Create Stateless EJB having EntityManager instance

@Stateless public class LibraryPersistentBean implements LibraryPersistentBeanRemote { //pass persistence unit to entityManager. @PersistenceContext(unitName="EjbComponentPU") private EntityManager entityManager; public void addBook(Book book) { entityManager.persist(book); } public List<Book> getBooks() { return entityManager.createQuery("From Books").getResultList(); } ... }

构建EJB模块后，我们需要一个无状态的bean，我们将在下一节要创建客户端来访问。

示例应用程序

让我们创建一个测试EJB应用程序来测试EJB的持久化机制。

EJBComponent (EJB Module)

Book.java

package com.tutorialspoint.entity; import java.io.Serializable; import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.EntityListeners; import javax.persistence.GeneratedValue; import javax.persistence.GenerationType; import javax.persistence.Id; import javax.persistence.Table; @Entity @Table(name="books") public class Book implements Serializable{ private int id; private String name; public Book(){ } @Id @GeneratedValue(strategy= GenerationType.IDENTITY) @Column(name="id") public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } }

LibraryPersistentBeanRemote.java

package com.tutorialspoint.stateless; import com.tutorialspoint.entity.Book; import java.util.List; import javax.ejb.Remote; @Remote public interface LibraryPersistentBeanRemote { void addBook(Book bookName); List<Book> getBooks(); }

LibraryPersistentBean.java

package com.tutorialspoint.stateless; import com.tutorialspoint.entity.Book; import java.util.List; import javax.ejb.Stateless; import javax.persistence.EntityManager; import javax.persistence.PersistenceContext; @Stateless public class LibraryPersistentBean implements LibraryPersistentBeanRemote { public LibraryPersistentBean(){ } @PersistenceContext(unitName="EjbComponentPU") private EntityManager entityManager; public void addBook(Book book) { entityManager.persist(book); } public List<Book> getBooks() { return entityManager.createQuery("From Book").getResultList(); } }

• As soon as you deploy the EjbComponent project on JBOSS, notice the jboss log.

• JBoss has automatically created a JNDI entry for our session bean -LibraryPersistentBean/remote.

• We'll using this lookup string to get remote business object of type -com.tutorialspoint.stateless.LibraryPersistentBeanRemote

JBoss应用服务器的日志输出

...
16:30:01,401 INFO  [JndiSessionRegistrarBase] Binding the following Entries in Global JNDI:
   LibraryPersistentBean/remote - EJB3.x Default Remote Business Interface
   LibraryPersistentBean/remote-com.tutorialspoint.stateless.LibraryPersistentBeanRemote - EJB3.x Remote Business Interface
16:30:02,723 INFO  [SessionSpecContainer] Starting jboss.j2ee:jar=EjbComponent.jar,name=LibraryPersistentBeanRemote,service=EJB3
16:30:02,723 INFO  [EJBContainer] STARTED EJB: com.tutorialspoint.stateless.LibraryPersistentBeanRemote ejbName: LibraryPersistentBean
16:30:02,731 INFO  [JndiSessionRegistrarBase] Binding the following Entries in Global JNDI:

   LibraryPersistentBean/remote - EJB3.x Default Remote Business Interface
   LibraryPersistentBean/remote-com.tutorialspoint.stateless.LibraryPersistentBeanRemote - EJB3.x Remote Business Interface
...   

EJBTester (EJB Client)

jndi.properties

java.naming.factory.initial=org.jnp.interfaces.NamingContextFactory java.naming.factory.url.pkgs=org.jboss.naming:org.jnp.interfaces
java.naming.provider.url=localhost

• These properties are used to initialize the InitialContext object of java naming service

• InitialContext object will be used to lookup stateless session bean

EJBTester.java

package com.tutorialspoint.test; import com.tutorialspoint.stateless.LibraryPersistentBeanRemote; import java.io.BufferedReader; import java.io.FileInputStream; import java.io.IOException; import java.io.InputStreamReader; import java.util.List; import java.util.Properties; import javax.naming.InitialContext; import javax.naming.NamingException; public class EJBTester { BufferedReader brConsoleReader = null; Properties props; InitialContext ctx; { props = new Properties(); try { props.load(new FileInputStream("jndi.properties")); } catch (IOException ex) { ex.printStackTrace(); } try { ctx = new InitialContext(props); } catch (NamingException ex) { ex.printStackTrace(); } brConsoleReader = new BufferedReader(new InputStreamReader(System.in)); } public static void main(String[] args) { EJBTester ejbTester = new EJBTester(); ejbTester.testEntityEjb(); } private void showGUI(){ System.out.println("**********************"); System.out.println("Welcome to Book Store"); System.out.println("**********************"); System.out.print("Options 
1. Add Book
2. Exit 
Enter Choice: "); } private void testEntityEjb(){ try { int choice = 1; LibraryPersistentBeanRemote libraryBean = LibraryPersistentBeanRemote)ctx.lookup("LibraryPersistentBean/remote"); while (choice != 2) { String bookName; showGUI(); String strChoice = brConsoleReader.readLine(); choice = Integer.parseInt(strChoice); if (choice == 1) { System.out.print("Enter book name: "); bookName = brConsoleReader.readLine(); Book book = new Book(); book.setName(bookName); libraryBean.addBook(book); } else if (choice == 2) { break; } } List<Book> booksList = libraryBean.getBooks(); System.out.println("Book(s) entered so far: " + booksList.size()); int i = 0; for (Book book:booksList) { System.out.println((i+1)+". " + book.getName()); i++; } } catch (Exception e) { System.out.println(e.getMessage()); e.printStackTrace(); }finally { try { if(brConsoleReader !=null){ brConsoleReader.close(); } } catch (IOException ex) { System.out.println(ex.getMessage()); } } } }

EJBTester is doing the following tasks.

• Load properties from jndi.properties and initialize the InitialContext object.

• In testStatefulEjb() method, jndi lookup is done with name - "LibraryStatefulSessionBean/remote" to obtain the remote business object (stateful ejb).

• Then user is shown a library store User Interface and he/she is asked to enter choice.

• If user enters 1, system asks for book name and saves the book using stateless session bean addBook() method. Session Bean is persisting the book in database via EntityManager call.

• If user enters 2, system retrieves books using stateful session bean getBooks() method and exits.

• Then another jndi lookup is done with name - "LibraryStatelessSessionBean/remote" to obtain the remote business object (stateless ejb) again and listing of books is done.

运行客户端访问EJB

在项目资源管理器中找到EJBTester.java。右键点击上EJBTester类，并选择 run file.

在Netbeans控制台验证以下输出。

run:
**********************
Welcome to Book Store
**********************
Options 
1. Add Book
2. Exit 
Enter Choice: 1
Enter book name: Learn Java
**********************
Welcome to Book Store
**********************
Options 
1. Add Book
2. Exit 
Enter Choice: 2
Book(s) entered so far: 1
1. learn java
BUILD SUCCESSFUL (total time: 15 seconds)

再次运行客户端来访问EJB。

访问EJB之前重新启动JBoss。

在项目资源管理器中找到EJBTester.java。右键点击上EJBTester类，并选择 run file.

在Netbeans控制台验证以下输出。

run:
**********************
Welcome to Book Store
**********************
Options 
1. Add Book
2. Exit 
Enter Choice: 1
Enter book name: Learn Spring
**********************
Welcome to Book Store
**********************
Options 
1. Add Book
2. Exit 
Enter Choice: 2
Book(s) entered so far: 2
1. learn java
2. Learn Spring
BUILD SUCCESSFUL (total time: 15 seconds)

• 上面显示的输出状态书存储在持久性存储，并从数据库中检索。